Abstract
We present a non-invasive approach for determining plasma parameters such as electron temperature and density inside a radio-frequency ion thruster (RIT) using optical emission spectroscopy (OES) in conjunction with principal component analysis (PCA). Instead of relying on a theoretical microscopic model of the plasma emission to extract plasma parameters from the OES, an empirical correlation is established on the basis of conducting simultaneous OES and Langmuir diagnostics. The measured reference spectra are simplified and a PCA is performed. The PCA results are correlated with the plasma parameters of the Langmuir measurements yielding a one-to-one correspondence. This correlation allows us to derive the plasma parameters by analysis of a non-invasively determined emission spectrum without additional Langmuir measurements. We show how the plasma parameters can be calculated from OES measurements using this correlation. Under the assumption that the electronic system thermalizes on much shorter time scales than the period of the RF signal driving the thruster, we can also use time-resolved spectral data to determine the time evolution of plasma parameters. In future, this method may contribute to shorter test and qualification times of RITs and other ion thrusters.
Highlights
Electric propulsion (EP) systems are nowadays commonly used on spacecrafts [1, 2]
We present a non-invasive approach for determining plasma parameters such as electron temperature and density inside a radio-frequency ion thruster (RIT) using optical emission spectroscopy (OES) in conjunction with principal component analysis (PCA)
The PCA yields a set of new orthogonal coordinate axes, i. e., the principal components PCE,i (i = 1,..., n), along which the spread in the data set is maximum, as they are the eigenvectors of the variance
Summary
Electric propulsion (EP) systems are nowadays commonly used on spacecrafts [1, 2]. Due to their high thrust to fuel consumption ratio as well as their large variety of different implementations and usable propellants, EP systems are a versatile alternative to their chemical counterparts in many in-orbit applications. We measure emission spectra simultaneously with the plasma parameters in a RIT like setup and correlate the results with the help of a principal component analysis (PCA)[8]. We measure a reference data set of emission spectra simultaneously with the plasma parameters in a RIT like setup operating with a propellant plasma, such as xenon. This correlation can be employed to extract plasma parameters non-invasively from plasmas operating under similar conditions as the reference plasma, for instance, inside a thruster For this purpose, an optical emission spectrum S (λ) is measured and expanded in terms of the PC of the reference data to yield its PC scores. If the sum of the variances of PC1 and PC2 is already close to the total variance, these two axes are sufficient to describe the main differences in the data
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